In housebuilding, it has become more and more common to use prefabricated members i.e. elements. The elements are manufactured in prefabrication factories, whereafter they are transported to the building site and fixed as a part of the frame of the building under construction. Typical elements manufactured under plant conditions are wooden nail plate trusses. Nail plate trusses are manufactured by placing the truss chords and members in their respective places on a horizontal stationary assembly table, whereafter they are connected by means of nail plates. The nail plates are placed in their position on both sides of the connecting point and pressed in their position by means of a movable hydraulic press. In a traditional method of manufacturing roof trusses the installation of the members and chords on their position on the assembly table, placing of the nail plates at the connecting points and the pressing of the nail plates in their position are conducted manually, which increases the manufacturing costs of the trusses. By employing the method of manufacturing nail plate trusses it is also possible to manufacture other wooden structures composed of rod-like members, such as wall frames.
Publication US 2008/0172983 A1 discloses a method and a system for assembling nail plate trusses. In this solution the truss moves along a horizontal assembly line at the assembly stage. The assembly line contains several stations for the different stages of assembling the truss. Furthermore, the system contains for instance a station for sawing the members, a station for positioning the chord memburs with respect to each other, a station for positioning the diagonal and vertical members with respect to the chord members, and a station for fastening the vertical members to the chord members by means of nail plate joints. The assembly line is long and the assembly of the truss takes place in horizontal position, wherein it requires a great deal of space. The act of moving the truss on the assembly line requires fastening of the diagonal and vertical members temporarily to the chord members by means of staples before making the final nail plate joints. This additional work stage decelerates the manufacture of the truss and increases the manufacturing costs.
Publication WO 2009/156559 A1 discloses a system for manufacturing roof trusses or the like. This system comprises a vertical assembly table containing grippers, on the support of which the members and chords of the truss are fastened. The grippers are fastened to beams which can be moved vertically and horizontally and which can be pivoted in various positions. The beams, in turn, are fastened to pillars that move along the rails of a transfer line during the assembly of the truss. This system contains a large number of moving parts, whose position with respect to each other must be known accurately during the entire manufacturing stage of the truss. For this reason the system has a complex structure and it is difficult to implement its control mechanism. The moving of the truss at the assembly stage reduces especially the dimension accuracy of the joints, and makes it difficult to remain within the manufacturing tolerances.